1. Field of the Invention
The present invention relates to a pneumatic tire, especially to a mud and snow type pneumatic tire which has improved driving and braking performances in running on mud and snow without increasing a lug groove area.
2. Description of the Prior Art
Conventionally, many of the tires classified as a mud and snow type employ a tread pattern including four circumferential direction main grooves provided in the tread surface, so that land portions are defined by the circumferential direction main grooves. The land portions are formed as ribs or blocks having lug grooves. Such tread pattern causes the tire to exhibit high braking and driving performance in running on mud and snow, and thereby to have excellent off-road performance.
It is said that the off-road performance can be effectively improved by increasing the lug groove component in the ribs or the blocks of the above tread pattern.
However, there is a recent demand for recreational vehicles (RV) and the like to have favorable driving stability not only in off-road running but in on-road running. In response to such demand, some tires have increased lug groove component in the ribs or the blocks of the above tread pattern. However, such increase lowers stiffness of the ribs or of the blocks, leading to low driving stability in on-road running.
Japanese patent application Kokai publication No. 2006-137239 proposes a tire designed to have improved braking and driving performance in running on snow without increasing the lug groove component. The tire has the following four circumferential direction main grooves provided in the tread surface: first circumferential direction main grooves provided on a center region and second circumferential direction main grooves provided on respective shoulder regions. The first circumferential direction main grooves are each provided with multiple circumferential direction small grooves extending in the tire circumferential direction in the groove side walls thereof. The second circumferential direction main grooves are each provided with multiple radial direction grooves extending in the tire radial direction in the groove side walls thereof. The circumferential direction small grooves reduce a frictional resistance between water and the groove walls. The radial direction small grooves increase the friction with snow columns formed in the respective grooves when the tire comes into contact with the snowy road surface. Thereby, braking and driving performance in running on snow is improved.
However, the configuration providing only the circumferential direction small grooves and the radial direction small grooves as described above provides limited improvement in the braking and driving performances in running on mud and snow, unless the lug groove component is increased. Accordingly, deterioration in the rib stiffness or the block stiffness cannot be avoided, and this technique is insufficient in achieving a good balance between the braking and driving performance in running on mud and snow as well as the driving stability in on-road running.